Printable polypropylene film having anti-fog properties

ABSTRACT

The invention relates to a biaxially oriented polypropylene multilayer film, which comprises at least one base layer (B), an intermediate layer (I) and a top layer (I), and which contains anti-fog additives. The anti-fog additive is only added to the intermediate layer (I) in a quantity ranging from 0.5 to 8 wt. % with regard to the weight of the intermediate layer, and the film contains a maximum of 0.5 to &lt;1 wt. % of anti-fog additives with regard to the total weight of the film. The film is suited for packaging moist goods.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage of International Application No.PCT/EP02/02758, filed on Mar. 13, 2002.

The invention relates to a biaxially oriented multilayer polypropylenefilm which includes at least one base layer B, an interlayer Z and a toplayer D and which comprises one or more antifogging additives.

Films having particular antifogging properties are known from the priorart. Antifogging properties are an important prerequisite in filmpackaging for readily perishable foods having a high moisture content,such as salad, fresh fruit, for example berry fruit, and vegetables.These are, for example, packaged directly in film bags as wrap packagingor in trays covered or wrapped with a transparent film. A film which issuitable for this purpose must have good transparency and suppresscondensation of the moisture in the form of drops on the film surface.

In films which have no or poor antifogging property, the moisturecontent of the packaged foods results in the formation of water dropsdue to condensation on the film surface. This drop formation impairsfirstly the transparency and secondly the shelf life of the packcontents. The drops do not adhere to the film surface to an unlimitedextent, and the moisture dripping down accelerates rotting and theformation of mould on the food. In addition, the drops on the filmsurface act like a focusing lens on incidence of light. Films havingantifogging properties prevent this damaging action of drop-formwater-vapor condensation.

The antifogging properties of the film can be achieved by coating thefilm with an antifogging solution. Alternatively, the incorporation ofantifogging additives into the film itself is known. Antifoggingadditives are generally bivalent compounds which have a nonpolaraliphatic region for anchoring in the polyolefin matrix and a polarhydrophilic region which is able to interact with water. This polarregion effects the incompatibility of antifogging additives withpolyolefin, promoting migration of the additives to the film surface.The polar region here faces outwards and is thus able to interact withwater. The antifogging additive reduces the contact angle and causes theformation of a uniform transparent water film on the film surface andthus prevents undesired drop formation.

This desired action can be achieved well and reliably on polyethylenefilms. By contrast, it proves to be significantly more difficult toproduce polypropylene films having good antifogging properties. Forunexplained reasons, the migration behavior of the antifogging additivesin polyethylene appears to be different from that in polypropylene.

EP 0739398B1 describes a film having antifogging properties. These areachieved through a combination of three different additives in at leastone outer layer. The film comprises from 1 to 5% by weight of thisthree-component combination.

W/O 97/13640 describes a multilayer film which comprises an antifoggingadditive in one of the outer layers. This antifoggingadditive-containing top layer is built up from polyethylene. The filmpreferably has three layers.

The polypropylene antifogging films known from the prior art aredisadvantageous with respect to the printability and/or transparency orthe hot-tack properties of the film. Furthermore, the known films areunsatisfactory with respect to the antifogging properties, in particularalso with respect to the long-term stability of these properties.

The object of the present invention was to provide a film having goodantifogging properties which are stable in the long term. At the sametime, the film should have very good optical properties, i.e. hightransparency and good gloss, and heat-sealing properties, in particulargood hot-tack. It is likewise required that the film be printable on atleast one side, if possible the outer side.

This object is achieved in accordance with the invention by a biaxiallyoriented multilayer polypropylene film which includes at least one baselayer B, an interlayer I and a top layer I and which comprisesantifogging additives, where the antifogging additive is only added tothe interlayer I in an amount of from 0.5 to 8% by weight, based on theweight of the interlayer, and the film comprises a maximum of from 0.5to <1% by weight of antifogging additives, based on the total weight ofthe film.

In accordance with the invention, the film includes at least threelayers and consists of a base layer B and an inner interlayer Z and atop layer D applied to the inner interlayer, in accordance with thelayer structure BZD.

For the purposes of the present invention, the base layer is the layerwhich has the greatest layer thickness and makes up at least 40%,preferably from 50 to 90%, of the total film thickness. Top layers arethe layers which form the outer layers. Interlayers are of courseprovided between other layers present, in general between the base layerand a top layer. The inner interlayer is provided on the side of thefilm which later faces the packaged product.

In a further embodiment, the film consists of a base layer B,interlayers Z applied to both sides, and top layers D applied to theinterlayers, i.e. a five-layer structure DZBZD. In a further preferredembodiment, the film consists of a base layer B, an interlayer Z appliedto one side thereof, and top layers D applied to the base layer and theinterlayer, in accordance with DBZD. If desired, these base structurescomprising three, four or five layers may comprise further interlayers.The interlayers Z and the top layers D may be built up from rawmaterials which are different from one another.

The base layer of the film generally comprises at least 90% by weight,preferably from 95 to 100% by weight, in particular from 98 to <100% byweight, in each case based on the base layer, of a propylene polymerdescribed below. No antifogging additive is added to this base layer.

It has been found that a polypropylene base layer without addition ofantifogging additive advantageously contributes to the desired propertyprofile of the film. Surprisingly, it has been found that theantifogging additives only migrate from the interlayer I to the firstinner surface of the film, and the base layer acts as an effectivemigration barrier to the antifogging additive. This barrier action is sopronounced that the second outer top layer can be corona- orflame-treated in order to improve the printability. In spite of thistreatment, the migration-promoting action of this corona or flametreatment does not arise. It was expected that corona treatment of thefilm on both sides would result in migration of the antifogging additiveto both surfaces of the film. In particular, this was to be expected atthe comparatively high concentrations of antifogging additive in theinterlayer. It is known in principle from comparable film formulationsin the prior art that an “empty” base layer can promote targetedmigration of migrating additives to a film surface. However, this hashitherto only been described for formulations in which the total contentof migrating additives is kept very low. However, such small amounts ofantifogging additives did not exhibit the desired action.

The propylene polymer of the base layer comprises at least >98 to 100%by weight of propylene. The corresponding comonomer content of from 0 toat most 2% by weight, based on the propylene, generally consists, ifpresent, of ethylene. Isotactic propylene homopolymer is preferred.

The propylene homopolymer generally has a melting point of from 140 to170° C., preferably from 150 to 165° C., and generally has a melt flowindex (measurement DIN 53 735 at a load of 21.6 N and 230° C.) of from1.5 to 20 g/10 min, preferably from 2 to 15 g/10 min. Then-heptane-soluble content of the isotactic polymer is generally from 1to 6% by weight, based on the polymer. If desired, the propylene polymerof the base layer may be peroxidically degraded.

In general, the base layer comprises conventional stabilizers andneutralizers in effective amounts in each case, and optionallylubricants and/or antistatics. All the following data in % by weightrelate to the weight of the base layer.

Stabilizers which can be employed are the conventional compounds whichhave a stabilizing action for ethylene, propylene and other alpha-olefinpolymers. They are added in an amount of between 0.05 and 2% by weight.Particularly suitable are phenolic stabilizers, alkali/alkaline earthmetal stearates and/or alkali/alkaline earth metal carbonates. Phenolicstabilizers are preferred in an amount of from 0.1 to 0.6% by weight, inparticular from 0.15 to 0.3% by weight, and having a molecular weight ofgreater than 500 g/mol. Pentaerythrityltetrakis-3-(3,5-di-tertiary-butyl-4-hydroxyphenyl)propionate or1,3,5-trimethyl-2,4,6-tris(3,5-di-tertiary-butyl-4-hydroxybenzyl)benzeneare particularly advantageous.

Neutralizers are preferably dihydrotalcite, calcium stearate and/orcalcium carbonate having a mean particle size of at most 0.7 μm, anabsolute particle size of less than 10 μm and a specific surface area ofat least 40 m/g.

Suitable lubricants for the antifogging film according to the inventionare higher aliphatic acid amides made from an amine and awater-insoluble monocarboxylic acid (so-called fatty acids) having from8 to 24 carbon atoms, preferably from 10 to 18 carbon atoms. Erucamide,stearamide and oleamide are preferred. Lubricants are usually added inan amount of from 0.1 to 0.3% by weight, based on the base layer.

Preferred antistatics are the essentially straight-chain and saturatedaliphatic, tertiary amines having an aliphatic radical having from 10 to20 carbon atoms which are substituted by hydroxy-(C1–C4)-alkyl groups,where N,N-bis(2-hydroxyethyl)alkylamines having from 10 to 20 carbonatoms, preferably from 12 to 18 carbon atoms, in the alkyl radical areparticularly suitable. Further suitable antistatics are monoesters ofglycerol and aliphatic fatty acids, where fatty acid radicals havingfrom 10 to 20 carbon atoms are preferred. Particular preference is givento glycerol monostearate. Antistatics are usually added in an amount offrom 0.1 to 0.3% by weight. In general, the addition of antistatics isnot necessary.

The polypropylene film according to the invention furthermore comprisesa first inner interlayer I applied to the base layer and optionally asecond interlayer II provided on the opposite side of the base layer.The inner interlayer I is built up from polymers of olefins having from2 to 10 carbon atoms, preferably propylene copolymers and/or terpolymerswhose propylene content is 70–98% by weight, preferably from 90 to <98%by weight, based on the propylene. In general, the interlayer Icomprises 92–99.5% by weight, preferably 95–99% by weight, of thesepropylene polymers.

Examples of olefinic polymers of this type are

-   a copolymer of-   ethylene and propylene or-   ethylene and 1-butylene or-   propylene and 1-butylene or-   a terpolymer of-   ethylene and propylene and 1-butylene or-   a blend or mixture of two or more of the said homopolymers,    copolymers and terpolymers,-   where, in particular, random ethylene-propylene copolymers having an    ethylene content of from 1 to 10% by weight, preferably from 2.5 to    8% by weight, or random propylene-1-butylene copolymers having a    butylene content of from 2 to 25% by weight, preferably from 4 to    20% by weight, in each case based on the total weight of the    copolymer, or-   random ethylene-propylene-1-butylene terpolymers having an ethylene    content of from 1 to 10% by weight, preferably from 2 to 6% by    weight, and a 1-butylene content of from 2 to 20% by weight,    preferably from 4 to 20% by weight, in each case based on the total    weight of the terpolymer, or-   a blend of an ethylene-propylene-1-butylene terpolymer and a    propylene-1-butylene copolymer having an ethylene content of from    0.1 to 7% by weight and a propylene content of from 50 to 90% by    weight and a 1-butylene content of from 10 to 40% by weight, in each    case based on the total weight of the polymer blend, are preferred.

The above-described copolymers and terpolymers employed in the innerinterlayer I generally have a melt flow index of from 1.5 to 30 g/10min, preferably from 3 to 15 g/10 min. The melting point is preferablyin the range from 120 to 140° C. The above-described blend of copolymersand terpolymers has a melt flow index of from 5 to 9 g/10 min and amelting point of from 120 to 150° C. All the melt flow indices indicatedabove are measured at 230° C. and a force of 21.6 N (DIN 53 735).

In accordance with the invention, the first, inner interlayer Icomprises from 0.5 to 8% by weight, preferably from 1 to 5% by weight,of antifogging additive, based on the weight of the interlayer. Theamount of antifogging additive in the interlayer is at the same timeselected in such a way that the total amount of antifogging additive,based on the film, is at least from 0.4 to <1% by weight, preferably0.6–<1% by weight. In accordance with the invention, this amount ofadditive is only added to the inner interlayer I.

Surprisingly, the specific formulation of the film with high additivecontents only in the interlayer I makes it possible to achieve variousdesired service properties of the film alongside one another. The filmexhibits the desired antifogging properties, in particular these aresurprisingly uniform and stable in the long term. The interlayer formsan additive depot from which the additive can continue to migrate over along time, so that drop formation on the films does not occur, evenafter several days, when these are used for the packaging of moistfoods. At the same time, the disadvantageous effects of the antifoggingadditives on the transparency and hot tack surprisingly do not occur. Inspite of good antifogging properties, the film exhibits good hot tackand very good transparency.

Antifogging additives are generally bivalent compounds which have anonpolar aliphatic region for anchoring in the polyolefin matrix and apolar hydrophilic region which is able to interact with water/aqueoussystems. This polar region causes the incompatibility of antifoggingadditives with polypropylene, which results in migration of theadditives to the film surface, where the polar region is directedoutwards and is thus able to interact with water. The antifoggingadditive reduces the contact angle and thus allows the formation of acontinuous transparent water film on the film surface.

Antifogging additives are thus surface-active additives which arepreferably employed as a combination of various substance classes.Suitable substances which are employed in combination with one anotheras antifogging additive are, for example, described in WO 97/22655,which is expressly incorporated herein by way of reference. Preferredcomponents of the combination are fatty acid esters and derivativesthereof, aliphatic alcohols and esters thereof, polyethoxylated aromaticalcohols, mono- or polyesterified sorbitol esters, mono- orpolyesterified glycerol esters, mixed glycerol esters, ethoxylatedsorbitan esters and ethoxylated amines. The above-mentioned esters aregenerally based on esterification of the respective acid or alcoholfunctions with medium- or long-chain fatty acids.

In a preferred embodiment, the interlayer I comprises anactive-ingredient combination from the three substance classes: glycerolesters, sorbitol esters and ethoxylated amines. In this embodiment, theinterlayer I comprises in total 3–5% by weight of the active-ingredientcombination from the said different substance classes, in each casebased on the weight of the interlayer I.

The antifogging additives can be incorporated into the film directly orvia a concentrate, a so-called masterbatch. Masterbatches of this typeare polyolefins which comprise the antifogging additives in an increasedconcentration compared with the film layer, generally 30–80% by weight,preferably 40–60% by weight of the additive, based on the total weightof the concentrate. The polyolefins of the masterbatch are generallystandard polyethylenes or polypropylenes as described for the film ofthe present invention.

Antifogging additives and corresponding masterbatches are prior art perse and are commercially available.

The polypropylene film according to the invention can have a secondinterlayer II comprising polymers of olefins having from 2 to 10 carbonatoms applied to the second surface of the base layer.

This second interlayer II preferably consists of 98–100% by weight ofthe polypropylene raw materials described for the base layer, where thepropylene homopolymer employed in the second interlayer II comprises atleast >98–100% by weight of propylene and generally has a melting pointof 140° C. or above, preferably from 150 to 170° C. Isotactichomopolypropylene having an n-heptane-soluble content of 6% by weight orless, based on the isotactic homopolypropylene, is preferred. Thehomopolymer generally has a melt flow index of from 1.5 g/10 min to 20g/10 min, preferably from 2.0 g/10 min to 15 g/10 min.

In a further embodiment, blends of the propylene homopolymer withpropylene copolymers or terpolymers are also possible. The copolymersand terpolymers which can be employed in such blends have already beendescribed as polymers for the inner interlayer I. Mixtures for theinterlayer II comprise a maximum of 30% by weight, preferably from 1 to10% by weight, based on the interlayer II, of the said copolymers orterpolymers.

Furthermore, in general both interlayers I and II additionally comprisethe stabilizers and neutralizers described for the base layer in thecorresponding amounts based on the weight of the interlayer. If desired,the interlayer II may additionally comprise conventional lubricants andantistatics. However, no antifogging additives are added to the secondinterlayer II.

The thickness of the interlayers I is generally in the range from 0.2 to10 μm, preferably in the range from 0.4 to 8 μm, in particular from 1 to6 μm. The thickness of the interlayer II is generally in the range from0.2 to 15 μm, preferably 1–10 μm.

The polypropylene film according to the invention furthermore comprisesa top layer, referred to below as inner top layer I, applied to theinner interlayer I. On use, it faces the pack contents. If desired,there is additionally a second top layer II provided on the oppositeside, i.e. on the second surface of the base layer or interlayer II.

The inner top layer I is generally built up from polymers of olefinshaving from 2 to 10 carbon atoms, preferably propylene copolymers orterpolymers whose propylene content is at least 50% by weight,preferably 70–98% by weight. In general, the inner top layer I comprises95–100% by weight, preferably 98–<100% by weight, of the propylenecopolymers or terpolymers described.

Examples of olefinic polymers of this type are

-   a copolymer of-   ethylene and propylene or-   ethylene and 1-butylene or-   propylene and 1-butylene or-   a terpolymer of-   ethylene and propylene and 1-butylene or-   a blend or mixture of two or more of the said homopolymers,    copolymers and terpolymers,-   where, in particular, random ethylene-propylene copolymers having an    ethylene content of from 1 to 10% by weight, preferably from 2.5 to    8% by weight, or random propylene-1-butylene copolymers having a    butylene content of from 2 to 25% by weight, preferably from 4 to    20% by weight, in each case based on the total weight of the    copolymer, or-   random ethylene-propylene-1-butylene terpolymers having an ethylene    content of from 1 to 10% by weight, preferably from 2 to 6% by    weight, and a 1-butylene content of from 2 to 20% by weight,    preferably from 4 to 20% by weight, in each case based on the total    weight of the terpolymer, or-   a blend of an ethylene-propylene-1-butylene terpolymer and a    propylene-1-butylene copolymer having an ethylene content of from    0.1 to 7% by weight and a propylene content of from 50 to 90% by    weight and a 1-butylene content of from 10 to 40% by weight, in each    case based on the total weight of the polymer blend, are preferred.

The above-described copolymers and terpolymers employed in the inner toplayer I generally have a melt flow index of from 1.5 to 30 g/10 min,preferably from 3 to 15 g/10 min. The melting point is in the range from120 to 140° C. The above-described blend of copolymers and terpolymershas a melt flow index of from 5 to 9 g/10 min and a melting point offrom 120 to 150° C. All the melt flow indices indicated above aremeasured at 230° C. and a force of 21.6 N (DIN 53 735).

The polypropylene film according to the invention may have a second toplayer II of propylene polymers applied to the base layer or theinterlayer II. This second top layer II consists (98–100% by weight),where present, of the propylene copolymers or terpolymers described forthe top layer I, or alternatively of the propylene homopolymersdescribed for the base layer, where the propylene homopolymer employedin the second top layer II consists predominantly (>98–100% by weight)of propylene and generally has a melting point of 140° C. or above,preferably from 150 to 170° C., where isotactic homopolypropylene havingan n-heptane-soluble content of 6% by weight or less, based on theisotactic homopolypropylene, is preferred. The homopolymer generally hasa melt flow index of from 1.5 g/10 min to 20 g/10 min, preferably from2.0 g/10 min to 15 g/10 min.

The suitable propylene copolymers or terpolymers, which comprise atleast 50% by weight of propylene units, have already been describedabove for the top layer I. These are likewise particularly suitable fortop layer II. This embodiment can also be corona- or flame-treated onthe surface of the top layer II.

In order to improve the printability of the top layer II, blends of theabove-described propylene homopolymer with copolymers or terpolymers canalso be employed. The copolymers and terpolymers used in such mixturesin the second top layer II have already been described for the inner toplayer I. In general, mixtures of this type for the top layer II comprisefrom 1 to 30% by weight, preferably from 1 to 20% by weight, of thecopolymers or terpolymers. In general, no antifogging additives areadded to the second top layer II.

The top layers I and II may comprise the stabilizers and neutralizersdescribed for the base layer in the corresponding amounts based on theweight of the corresponding layer. In a preferred embodiment, the toplayers I and/or II comprise antiblocking agents described below. Noantifogging additives are added to either the first inner top layer I orthe second top layer II, like the base layer and the second interlayerII. In accordance with the invention, the antifogging additives are onlyadded to the inner interlayer I, which functions as additive depot.

It goes without saying that small amounts of antifogging additive may bepresent in the other layers of the film, for example due to conventionalprocessing of film regrind. However, this amount must be kept so smallthat interfering amounts do not migrate to the second outer surface ofthe film. In general, the base layer should comprise less than 1% byweight of these antifogging additives via film regrind. In such anamount, the antifogging additives do not have a disadvantageous effecton the desired film properties in the base layer.

Suitable antiblocking agents for the top layers I and/or II areinorganic additives, such as silicon dioxide, calcium carbonate,magnesium silicate, aluminium silicate, calcium phosphate and the likeand/or incompatible organic polymers, such as polyamides, polyesters,polycarbonates and the like, preferably benzoguanamine-formaldehydepolymers, silicon dioxide and calcium carbonate. The effective amount ofantiblocking agent, preferably SiO2, is in the range from 0.1 to 2% byweight, preferably from 0.1 to 0.8% by weight. The mean particle size isbetween 1 and 6 μm, in particular 2 and 5 μm, where particles having aspherical shape, as described in EP-A-0 236 945 and DE-A-38 01 535, areparticularly suitable.

The thickness of the top layer(s) I and II is generally greater than 0.2μm and is preferably in the range from 0.4 to 3 μm, in particular from0.5 to 1.5 μm. The thickness of the respective top layers may beselected independently of one another.

The total thickness of the polypropylene film according to the inventioncan vary within broad limits and depends on the intended use. It ispreferably from 4 to 120 μm, in particular from 5 to 80 μm, preferablyfrom 7 to 50 μm, with the base layer making up from about 40 to 95% ofthe total film thickness.

The invention furthermore relates to a process for the production of thepolypropylene film according to the invention by the coextrusionprocess, which is known per se.

In this process, firstly, as usual in the coextrusion process, thepolymer or polymer mixture of the individual layers is compressed andliquefied in an extruder, where any additives added may already bepresent in the polymer or in the polymer mixture or are incorporated asa masterbatch. The melts are then forced simultaneously through aflat-film die (slot die), and the extruded multilayer film is taken offon one or more take-off rolls, during which it cools and solidifies.

The resultant film is then stretched longitudinally and transversely tothe extrusion direction, resulting in orientation of the moleculechains. The longitudinal stretching is advantageously carried out withthe aid of two rolls running at different speeds in accordance with thetarget stretching ratio, and the transverse stretching is advantageouslycarried out with the aid of a corresponding tenter frame. Thelongitudinal stretching ratios are in the range from 4 to 9, preferablyfrom 4.5 to 8.5. The transverse stretching ratios are in the range from5 to 10, preferably from 6 to 9.

The biaxial stretching of the film is followed by heat-setting (heattreatment) thereof, during which the film is held at a temperature offrom 60 to 160° C. for about 0.1 to 20 seconds. The film is subsequentlywound up in a conventional manner using a wind-up device.

It has proven particularly favorable to hold the take-off roll or rollsby means of which the extruded film is cooled and solidified at atemperature of from 10 to 100° C., preferably from 20 to 70° C., bymeans of a heating and cooling circuit.

The temperatures at which longitudinal and transverse stretching arecarried out can vary in a relatively large range and depend on thedesired properties of the film. In general, the longitudinal stretchingis preferably carried out at from 80 to 150° C. and the transversestretching preferably at from 120 to 180° C.

After the biaxial stretching, one or both surface(s) of the film is(are) preferably plasma-, corona- or flame-treated by one of the knownmethods. The treatment intensity is generally in the range from 36 to 50mN/m, preferably from 38 to 45 mN/m. In the case of treatment on oneside, it is possible to treat only the inner surface of the film.

In preferred embodiments, however, either both surfaces of the film orthe surface of the top layer II (i.e. the outer film side) are treated.It was originally expected that single-sided corona treatment on thesurface of the top layer I (i.e. the inner film side) would have aparticularly advantageous effect on the film properties since amigration-promoting action is ascribed to corona treatment. It has beenfound that this supporting action is surprisingly unnecessary. Theantifogging additives migrate to the “correct” inner film surface evenon treatment on both sides or on treatment on the outer film side. Inaddition, it has been found that single-sided corona treatment on theinner film side results in interfering taste impairment of the packagedproduct in the case of individual antifogging combinations. Thus,although single-sided “inner” corona treatment is possible, it is not,however, necessary and is also not always advantageous.

The corona treatment is advantageously carried out by passing the filmbetween two conductor elements serving as electrodes, with such a highvoltage, usually an alternating voltage (from about 5 to 20 kV and from5 to 30 kHz), being applied between the electrodes that spray or coronadischarges are able to occur. The spray or corona discharge causes theair above the film surface to ionize and react with the molecules of thefilm surface, resulting in the formation of polar inclusions in theessentially nonpolar polymer matrix.

The multilayer film according to the invention is distinguished by itsexcellent suitability as antifogging film for the packaging of productshaving a high moisture content. It has been found that the multilayerstructure in combination with the particular formulation of theindividual layers ensures the advantageous action of the antifoggingadditives known per se, but at the same time their disadvantageouseffect, such as hazing of the film, poorer printability, impaired hottack, is avoided.

Surprisingly, the exclusive formulation of the inner interlayer I withcomparatively high concentrations of antifogging additives is sufficientto achieve constantly good antifogging properties of the film. Theinterlayer I functions as additive depot and allows sufficiently fast,but also long-lasting and targeted migration of the antifogging additiveto the inner top layer I. A consistent antifogging property which isconstant over time is thus ensured.

It has been found that, due to the film composition according to theinvention, the antifogging additives do not significantly increase thehaze of the film. This is vital for a transparent packaging film whichis intended to present the pack contents in a visually attractivemanner.

Surprisingly, high additive concentrations only in the interlayer Iachieve better results with respect to the antifogging properties thanthe combination of antifogging additives in the base layer and the toplayer.

Surprisingly, the base layer forms an effective barrier to migration ofthe antifogging additive to the surface of the top layer II. Thisbarrier action is retained even on surface treatment of the top layer IIby corona or flame. Thus, the film can be surface-treated on one or bothsides, with the antifogging additive nevertheless migrating only to thesurface of the inner top layer I. The outer surface can, if necessary,be provided with a print. In addition, the film has better hot-tackproperties than antifogging films of the prior art. A film is thusprovided which has a particularly advantageous property combination. Thefilm has antifogging properties which are particularly stable anduniform in the long term and still has very good transparency, goodhot-tack properties and can be printed in the desired manner on theouter side II.

The invention is explained in greater detail by the following examples.

EXAMPLE 1

A transparent four-layer film consisting of the base layer B, an innerinterlayer Z1, an inner top layer D1 and an outer top layer D2 with atotal thickness of 30 μm was produced by coextrusion and subsequentstepwise orientation in the longitudinal and transverse directions. Theouter top layer D2 had a thickness of 0.7 μm, the inner interlayer Z1had a thickness of 4 μm and the inner top layer D1 had a thickness of1.0 μm. Both top layers of the film were corona-treated. The totalcontent of antifogging additives in the film was 0.67% by weight, basedon the total weight of the film. The layers had the followingcomposition:

Base Layer B:

-   99.84% by weight of propylene homopolymer having a melting point of    165° C. and a melt flow index of 3.4 g/10 min and a chain    isotacticity index of 94%-   0.03% by weight of neutralizer (CaCO3)-   0.13% by weight of stabilizer (Irganox)    Inner Interlayer I:-   95.0% by weight of random copolymer of ethylene and propylene having    a melt flow index of 6.0 g/10 min and an ethylene content of 6% by    weight, based on the copolymer-   5.0% by weight of antifogging additive combination comprising    -   ⅓—sorbitan monostearate    -   ⅓—ethoxylated sorbitan oleate    -   ⅓—glycerol dioleate        Inner Top Layer I:-   99.5% by weight of random copolymer of ethylene and propylene having    a melt flow index of 6.0 g/10 min and an ethylene content of 6% by    weight, based on the copolymer-   0.5% by weight of SiO2 as antiblocking agent having a mean particle    size of 4 μm    Outer Top Layer II:-   89.7% by weight of propylene homopolymer having a melting point of    165° C. and a melt flow index of 3.4 g/10 min and a chain    isotacticity index of 94%, with 0.03% by weight of neutralizer    (CaCO3) and 0.13% by weight of stabilizer (Irganox)-   10.0% by weight of random copolymer of ethylene and propylene having    a melt flow index of 6.0 g/10 min and an ethylene content of 6% by    weight, based on the copolymer-   0.3% by weight of SiO2 as antiblocking agent having a mean particle    size of 4 μm

The production conditions in the individual process steps were:

Extrusion: Temperatures Base layer B: 260° C. Interlayer 21: 250° C. Toplayer D1: 250° C. Top layer D2: 250° C. Temperature of the  20° C.take-off roll Longitudinal Temperature: 110° C. stretching: Longitudinal1:5 stretching ratio: Transverse Temperature: 165° C. stretching:Transverse 1:9 stretching ratio: Setting: Temperature: 140° C.Convergence: 10% Print pretreatment Top layer I and II Corona 10,000V/10,000 Hz

The transverse stretching ratio of 1:9 is an effective value. Thiseffective value is calculated from the final film width B reduced bytwice the hem width b, divided by the width of the longitudinallystretched film C, likewise reduced by twice the hem width b.

The film was produced without problems and was distinguished byexcellent properties:

-   -   very good printability on the outer surface and    -   good hot-tack properties and    -   very good antifogging properties

EXAMPLE 2

Analogously to Example 1, a transparent five-layer film with anadditional outer interlayer II was produced. This second interlayer IIwas located between the base layer B and the outer top layer II andconsisted of a propylene homopolymer having a melting point of 165° C.and a melt flow index of 3.4 g/10 min and a chain isotacticity index of94%.

The total thickness of the film was 30 μm. The outer top layer I had athickness of 0.7 μm, the outer interlayer II had a thickness of 2 μm,the inner interlayer I had a thickness of 4 μm and the inner top layer Ihad a thickness of 1.0 μm. The total content of antifogging additives inthe film was 0.67% by weight, based on the total weight of the film.

EXAMPLE 3

As in Example 2, a transparent five-layer film was produced. In contrastto Example 2, the copolymer in the inner interlayer I and the inner toplayer I was replaced by a random ethylene-propylene-butylene terpolymer.The random ethylene-propylene-butylene terpolymer has an ethylenecontent of 3% by weight and a butylene content of 7% by weight(remainder propylene) and was stabilized with 0.07% of both Irganox 1010and Irgafos 168 and 0.03% of DHT. The melt flow index is 7.3 g/10 min.The total content of antifogging additives in the film was 0.67% byweight, based on the total weight of the film. This enabled the hot-tackproperties of the film to be additionally improved.

EXAMPLE 4

As in Example 3, a transparent five-layer film was produced. In contrastto Example 3, 20% of a random copolymer of ethylene and propylene havinga melt flow index of 6.0 g/10 min and an ethylene content of 6% byweight, based on the copolymer, were admixed with the outer interlayerII. The content of propylene homopolymer was reduced correspondingly.The total content of antifogging additives in the film was 0.67% byweight, based on the total weight of the film. In spite of the admixingof copolymer in the outer interlayer II, the antifogging properties ofthe film remained constantly good.

EXAMPLE 5

As in Example 4, a transparent five-layer film was produced. In contrastto Example 3, the concentration of antifogging additives in the innerinterlayer I was reduced from 5% by weight to 3% by weight. The totalcontent of antifogging additives in the film was now only 0.40% byweight, based on the total weight of the film.

In spite of the reduction in the antifogging concentration in theinterlayer I, a film having good antifogging properties was produced.

COMPARATIVE EXAMPLE 1

As described in Example 1, a transparent four-layer film was produced bycoextrusion and subsequent stepwise orientation in the longitudinal andtransverse directions. In contrast to Example 1, 1% by weight of thesame antifogging additive were incorporated into each of the base layerB and the inner interlayer I and the inner top layer I. The totalcontent of antifogging additives thus increases to 0.98% by weight,based on the total weight of the film. In addition, only the inner toplayer I was corona-treated. The layers had the following composition:

Base Layer B:

-   98.84% by weight of propylene homopolymer having a melting point of    165° C. and a melt flow index of 3.4 g/10 min and a chain    isotacticity index of 94%-   0.03% by weight of neutralizer (CaCO3)-   0.13% by weight of stabilizer (Irganox)-   1.0% by weight of antifogging additive as in Example 1    Inner Interlayer I:-   99.0% by weight of propylene homopolymer having a melting point of    165° C. and a melt flow index of 3.4 g/10 min and a chain    isotacticity index of 94%, with 0.03% by weight of neutralizer    (CaCO3) and 0.13% by weight of stabilizer (Irganox)-   1.0% by weight of antifogging additive as in Example 1    Inner Top Layer I:-   98.5% by weight of random copolymer of ethylene and propylene having    a melt flow index of 6.0 g/10 min and an ethylene content of 6% by    weight, based on the copolymer-   0.5% by weight of SiO2 as antiblocking agent having a mean particle    size of 4 μm-   1.0% by weight of antifogging additive as in Example 1    Outer Top Layer II:-   99.7% by weight of propylene homopolymer having a melting point of    165° C. and a melt flow index of 3.4 g/10 min and a chain    isotacticity index of 94%, with 0.03% by weight of neutralizer    (CaCO3) and 0.13% by weight of stabilizer (Irganox)-   0.3% by weight of SiO2 as antiblocking agent having a mean particle    size of 4 μm

In spite of the increase in the concentration of antifogging additives,a film having significantly poorer antifogging and hot-tack propertieswas obtained which in addition could not be printed on the outer sideowing to the lack of corona treatment.

COMPARATIVE EXAMPLE 2

Analogously to Example 2, a transparent five-layer film was produced. Incontrast to Example 2, 1% by weight of antifogging additives wereincorporated into each of the base layer B, the inner interlayer I andthe inner top layer I. In addition, the random copolymer in the innerinterlayer I was replaced by a propylene homopolymer in order toadditionally promote migration of the antifogging additives to the innertop layer I.

The total content of antifogging additives thus increases to 0.98% byweight, based on the total weight of the film. The film iscorona-treated on both sides. The composition of the inner interlayer Iand the outer top layer II were as follows:

Inner Interlayer I:

-   99.0% by weight of propylene homopolymer having a melting point of    165° C. and a melt flow index of 3.4 g/10 min and a chain    isotacticity index of 94%, with 0.03% by weight of neutralizer    (CaCO3) and 0.13% by weight of stabilizer (Irganox)-   1.0% by weight of antifogging additive combination as in Example 1    Outer Top Layer II:-   99.7% by weight of propylene homopolymer having a melting point of    165° C. and a melt flow index of 3.4 g/10 min and a chain    isotacticity index of 94%, with 0.03% by weight of neutralizer    (CaCO3) and 0.13% by weight of stabilizer (Irganox)-   0.3% by weight of SiO2 as antiblocking agent having a mean particle    size of 4 μm

In spite of the increase in the concentration of antifogging additives,a film having significantly poorer antifogging and hot-tack propertieswas obtained, and in addition the printability is reduced.

COMPARATIVE EXAMPLE 3

A film was produced as described in Example 3. In contrast to Example 3,1% by weight of antifogging additives were incorporated into both theinner interlayer I and the inner top layer I. The total content ofantifogging additive thus drops to 0.17% by weight, based on the totalweight of the film.

Owing to the low antifogging concentration, the film has significantdeficiencies with regard to antifogging properties and hot-tack.

The following table shows the most important film properties of theexamples and comparative examples:

Antifogging Hot tack at 140° C. properties [mm] Hot-fog Cold-fog Toplayer I to top layer Examples test test Printability I01 + + + + + + + + + + + 02 + + + + + + + + + + +03 + + + + + + + + + + + + 04 + + + + + + + + + + + +05 + + + + + + + + + + + C 01 + + + + − + C 02 + + + + + + C 03− + + + + + Assessment criteria: + + + very good + + good + poor − verypoor

The following measurement methods were used to characterize the rawmaterials and the films:

Printability:

The corona-treated films were printed 14 days after production(short-term assessment) and 6 months after production (long-termassessment). The ink adhesion was assessed by means of the adhesive-tapetest. If little ink could be removed by means of an adhesive tape, theink adhesion was assessed as moderate and if a significant amount of inkwas removed, it was assessed as poor.

Antifogging Properties:

The antifogging properties of the film were determined by means of thecold-fog test and hot-fog test.

“Cold-Fog” Test:

A 250 ml beaker is filled with 200 ml of water and covered with theantifogging film in such a way that the inner top layer I of the film isfacing the water. The beaker is stored at a constant 4° C. in aconditioning cabinet. The antifogging film is assessed at certain timeintervals (5 min, 10 min, 20 min, 30 min, 1 h, 2 h, 3 h, 4 h, 1 d, 2 d,3 d, 4 d and 5 d) with regard to its antifogging action, i.e. theprevention of drop-form formation of condensed water.

Assessment criteria:

+ + + very good film transparent, no drop formation at all + + good filmtransparent, non-uniform water film + poor poor transparency due tolarge drops − very poor no transparency due to many small drops“Hot-Fog” Test:

A 250 ml beaker is filled with 50 ml of water and covered with theantifogging film in such a way that the inner top layer I of the film isfacing the water. The beaker is stored in a water bath at a constant 60°C. The antifogging film is assessed at certain time intervals (1 min, 2min, 5 min, 10 min, 20 min, 30 min, 1 h, 2 h, 3 h and 4 h) with regardto its antifogging action, i.e. the prevention of the formation ofcondensed water. The assessment criteria are analogous to the cold-fogtest.

Hot Tack

The term hot tack denotes the strength of a heat-seal seam directlyafter the sealing operation, i.e. still in the hot/warm state (hot seamstrength). To this end, a film strip with a width of 40 mm and a lengthof 600 mm is passed between the sealing jaws of a heat-sealing unit withthe aid of a spatula and held without touching the sealing jaws. Thefilm strip is weighted at the other end with a 200 g loading weight. Theheat sealing is generally carried out at 130–150° C. with a sealingpressure of 30 N/cm² and a sealing time of 0.5 s. When the sealing jawsclose, the spatula is immediately removed so that the seal seam isweighted with the loading weight at a 180° angle immediately after theopening of the sealing jaws in order to be able to test the seal seamstrength better. The mm by which the seal seam is raised are measured.

Melt Flow Index

The melt flow index was measured in accordance with DIN 53 735 at a loadof 21.6 N and 230° C.

Melting Point:

DSC measurement, maximum of the melting curve, heating rate 20 DEGC/min.

Haze:

The haze of the film was measured in accordance with ASTM-D 1003–52.

Gloss:

The gloss was determined in accordance with DIN 67 530. The reflectorvalue was measured as optical parameter for the surface of a film. Inaccordance with the ASTM-D 523–78 and ISO 2813 standards, the angle ofincidence was set at 60 DEG or 85 DEG. A light beam hits the planar testsurface at the set angle of incidence and is reflected or scatteredthereby. The light beams hitting the photoelectronic receiver aredisplayed as proportional electrical quantity. The measurement value isdimensionless and must be quoted together with the angle of incidence.

Surface Tension:

The surface tension was determined by the so-called ink method (DIN 53364).

1. A biaxially oriented multilayer polypropylene film comprising: atleast one base layer B; a top layer I; and an interlayer I between saidtop layer I and said base layer B, said interlayer I including at leastone antifogging additive in an amount of from 0.5% to 8% the weight ofthe interlayer I, and wherein the film has a weight and said antifoggingadditive comprises 0.5% to less than 1% of said weight and wherein noantifogging additive is added to said base layer B or said top layer I.2. The multilayer polypropylene film of claim 1 wherein the base layer Bincludes a propylene homopolymer in an amount of at least 90% the weightof the base layer B and wherein said propylene homopolymer is built upfrom greater than 98% by weight of propylene units.
 3. The multilayerpolypropylene film of claim 1, wherein the interlayer I comprisesapproximately 92 to 99.5% by weight of one of propylene copolymer or apropylene terpolymer, wherein each of said propylene copolymer and saidpropylene terpolymer are built up from approximately 70 to 99% by weightof propylene units.
 4. The multilayer polypropylene film of claim 1,wherein the antifogging additive consists of at least two componentsselected from the group consisting of: fatty acid esters, aliphaticalcohols, ethoxylated aromatic alcohols, mono- or polyesterifiedsorbitol esters, mono- or polyesterified glycerol esters, ethoxylatedsorbitan esters and ethoxylated amines.
 5. The multilayer polypropylenefilm of claim 1, wherein the antifogging additive includes at least 3components selected from the group consisting of: fatty acid esters,aliphatic alcohols, ethoxylated aromatic alcohols, mono- orpolyesterified sorbitol esters, mono- or polyesterified glycerol esters,ethoxylated sorbitan esters and ethoxylated amines.
 6. The multilayerpolypropylene film of claim 1, wherein the antifogging additive consistsof a mixture of glycerol esters sorbitol esters and ethoxylated amines.7. The multilayer polypropylene film of claim 1, wherein the film has aninterlayer II which comprises 98 to 100% by weight of a propylenehomopolymer having a propylene content of greater than 98% by weight andwherein said base layer B includes two opposing sides, said interlayer Ibeing on a first side and said interlayer II being on a second and saidinterlayer II being disposed between said base layer B and a top layerII.
 8. The multilayer polypropylene film of claim 1 further including aninterlayer II comprising a maximum of 70% by weight of a propylenehomopolymer having a propylene content of greater than 98% by weight andat most 30% by weight of one of propylene copolymer or propyleneterpolymer and wherein said base layer B includes two opposing sides,said interlayer I being on a first side and said interlayer II being ona seecond side and said interlayer II being disposed between said layerB and a top layer II.
 9. The multilayer polypropylene film of claim 1,wherein the top layer I comprises 95 to 100% by weight of one of apropylene copolymer or a propylene terpolymer and wherein each of saidpropylene copolymer and said propylene terpolymer have a propylenecontent from 70 to 98% by weight.
 10. The multilayer polypropylene filmof claim 1, further including a top layer II including at least 98% byweight of a propylene homopolymer having a propylene content of greaterthan 98% by weight.
 11. The multilayer polypropylene film of claim 1further including a top layer II which comprises a maximum of 70% byweight of a propylene homopolymer having a propylene content of greaterthan 98% by weight and at most 30% by weight of one of a propylenecopolymer or a propylene terpolymer.
 12. The multilayer polypropylenefilm of claim 11, wherein the top layer II has a surface and the film isprinted on the surface of the top layer II.
 13. The multilayerpolypropylene film of claim 1 wherein said film includes two sides andwherein said film is formed by the process of treating said sides with aprocess selected from the group consisting of corona-treating,flame-treating and plasma-treating.
 14. The multilayer polypropylenefilm of claim 1, further including a top layer II having a surface thatis treated with a process selected from the group consisting ofcorona-treating, flame-treating and plasma-treating.
 15. A process ofusing the multilayer polypropylene film of claim 1 as packaging film forthe packaging of products having a high water content, characterized inthat the top layer I faces the products.
 16. The multilayerpolypropylene film of claim 1 further including a top layer II having asurface, and wherein a transparent adhesive is applied to the surface ofthe top layer II for adhering the film to a transparent sheet ofmaterial.
 17. The multilayer polypropylene film of claim 1 furtherincluding a top layer II having a surface, and wherein an adhesionpromoter is applied to the surface of the top layer II for adhering thefilm to transparent sheets of material.
 18. The multilayer polypropylenefilm of claim 1, wherein the interlayer I comprises approximately 92 to99.5% by weight of a propylene copolymer, propyleneterpolymer or amixture thereof wherein each of said propylene copolymer and saidpropylene terpolymer are built up from approximately 70 to 99% by weightof propylene units.
 19. The multilayer polypropylene film of claim 1further including a interlayer II comprising a maximum of 70% by weightof a propylene homopolymer having a propylene content of greater than98% by weight and at most 30% by weight of a propylene copolymerpropylene terpolmer or a mixture thereof and wherein said base layer Bincludes two opposing sides, said interlayer I being on a first side andsaid interlayer II being on a second side.
 20. The multilayerpolypropylene film of claim 1, wherein the top layer I comprises 95 to100% by weight of a propylene copolymer propylene terpolymer or amixture thereof and wherein each of said propylene copolymer and saidpropylene terpolymer have a propylene content of from 70 to 98% byweight.
 21. The multilayer polypropylene film of claim 1 furtherincluding a top layer II which comprises a maximum of 70% by weight of apropylene homopolymer having a propylene content of greater than 98% byweight and at most 30% by weight of a propylene copolymer, propyleneterpolymer or a mixture thereof.
 22. A biaxially oriented multilayerpolypropylene film comprising: at least one base layer B comprising atleast 90% by weight of a propylene homopolymer built up from greaterthan 98% by weight of propylene units; a top layer I; and an interlayerI including antifogging additives in an amount of 0.5 to 8% by weight ofthe interlayer, and wherein the antifogging additives comprise 0.5% toless than 1% by weight of the combined weights of the base layer B, thetop layer I, and the interlayer I and no antifogging additive is addedto said base layer B or said top layer I.
 23. A biaxially orientedmultilayer polypropylene film comprising: at least one base layer B; atop layer I; and an interlayer I including antifogging additives in anamount of 0.5 to 8% by weight of the interlayer, and wherein theantifogging additives comprise 0.5 to less than 1% by weight of thecombined weights of the base layer B, the top layer I, and theinterlayer I and wherein the antifogging additives consist of at leasttwo components selected from the group consisting of fatty acid esters,aliphatic alcohols, ethoxylated aromatic alcohols, mono- orpolyesterified sorbitol esters, mono- or polyesterified glycerol esters,ethoxylated sorbitan esters and ethoxylated amines and wherein noantifogging additive is added to said base layer B and said top layer I.